1. Field of the Invention
The invention relates to position demodulating method and circuit of a disk apparatus, for demodulating a position of a head in order to position the head to a target position. More particularly, the invention relates to position demodulating method and circuit of a disk apparatus, for calculating a decoded position from a position signal of a disk which was read by a head.
2. Description of the Related Arts
A disk apparatus for reading a disk medium by a head comprises: a disk on which data is recorded; a motor for rotating the disk; the head for recording ad reproducing information on the disk; and an actuator for moving the head to a target position. As typical apparatuses, there are a magnetic disk apparatus known as a hard disk drive (HDD) and an optical disk apparatus known as a DVD-ROM or an MO.
In the magnetic disk apparatus, as shown in FIG. 1, position information 202 for detecting a position of a head has been recorded on a disk 200 at regular intervals in the circumferential direction from the inner rim to the outer rim. As shown in FIG. 2, the position information 202 comprises: a servo mark 204; a track number 206 using a gray code; an index 208; and offset information 210 including burst patterns corresponding to position signals PosA, PosB, PosC, and PosD. The present position of the head can be recognized by using the track number 206 and offset information 210 in the position information.
A read signal of the position information from the head has a signal waveform as shown in FIG. 3 and is inputted to a position detecting circuit. In the position detecting circuit, a servo mark signal 212 is detected, a start time of the position signal is recognized, the track number is extracted from a gray code signal 214, and position offset values are extracted from burst signals Ea, Eb, Ec, and Ed corresponding to the position signals PosA, PosB, PosC, and PosD and stored into a memory, respectively. The position detecting circuit is connected to an MCU (Main Control Unit) or a DSP (Digital Signal Processor). The MCU reads out the value from the memory and demodulates the present head position by calculations. The MCU calculates a value of a current which is supplied to an actuator in accordance with the present position. The calculated current value is set into a DAC. An output of the DAC is supplied to a current amplifier and a current is supplied to a motor (VCM) of the actuator. The MCU is connected to the outside of the apparatus via an HDC (Hard Disk Controller). Usually, a computer is provided for the outside of the apparatus and a command to record or reproduce the data is transmitted from the computer. The MCU analyzes the transmitted command and converts it into the actual position on the disk, thereby forming a target position and moving the actuator and the magnetic head to the target position.
According to a conventional position demodulating method, when the head reads out the burst patterns PosA, PosB, PosC, and PosD recorded in a zigzag shape on the magnetic disk as shown in FIG. 2 so as to have signal waveforms shown in FIG. 4, areas of the signals or areas of the absolute values are obtained by the position detecting circuit. Two position signals PosN and PosQ are calculated as follows by using the position signals PosA, PosB, PosC, and PosD detected from the four burst signals Ea, Eb, Ec, and Ed.PosN=PosA−PosBPosQ=PosC−PosD
FIG. 5 shows the position signals PosN and PosQ obtained by calculations. The position signal PosQ is arranged at a position where a phase is deviated from that of the position signal PosN by ¼ track. At this time, in each of the eight divided intervals, relations among the track numbers recorded on the medium, their detection errors, and the position signals PosN and PosQ are as shown in a table of FIG. 6. That is, the signal of the smaller absolute value between the position signals PosN and PosQ is used and the position is demodulated from the corresponding calculating expression. However, there is a case where a gray number (or track number) on the disk is erroneously read at a position near a switching point of the position signals PosN and PosQ. Therefore, if it is determined that the gray number was erroneously read, demodulating calculations shown in FIG. 7 are executed. Demodulating expressions obtained by logically compressing two tables as shown in FIGS. 6 and 7 as mentioned above can be expressed as a program of the C language as follows. G(x) denotes a position sensitivity gain at a near position x, that is, a gain for converting the value detected from the position signal PosN or PosQ by the DAC into an actual track unit. It is assumed that the demodulated gray number and track number are equal.PosN=PosA−PosB;PosQ=PosC−PosD;                Track=Gray;        
if(abs(PosN) <= abs(PosQ)){Position = −  sgn(PosQ)*G(x)*PosN + Track;if(sgn(PosQ)*even(Track) > 0.0)Position += sgn(PosQ)*sgn(PosN)*1.0;}else{Position = sgn(PosN)*(G(x)*PosQ + even(Track)*0.5) + Track;}
A relation between the decoded positions obtained by the demodulation calculating program and the actual position shows linear characteristics as shown in FIG. 8. The demodulation calculating program is expressed by a circuit as shown in FIG. 9. A demodulating circuit 218 receives the position signals read by the head and demodulates the track number and the position signals PosA, PosB, PosC, and PosD. Adders 220 and 222 calculate the position signals PosN and PosQ, respectively.PosN=PosA−PosBPosQ=PosC−PosDMultipliers 226 and 228 obtain the prestored position sensitivity gains with reference to a position sensitivity gain file 230 according to the track number and multiply the position signals PosN and PosQ by the obtained gains. A position calculating circuit 232 compares the position signals PosN and PosQ in which position sensitivities have been corrected by the multipliers 226 and 228 and obtains position information Pos1 by the calculating expression of the corresponding interval in the demodulation table of FIG. 6. Finally, the track number, an offset, and the position Pos1 are added by an adder 234 and a decoded position is outputted. Details of the position demodulation have been disclosed in JP-A-8-195044.
Timing when the decoded position can draw a straight line as shown in FIG. 8 is timing when the position sensitivity gain can be perfectly obtained. However, a measurement error occurs when the position sensitivity gain is measured. An amount of such a measurement error is equal to 10% or more in accordance with circumstances. One of factors of the measurement error is measuring precision. The position sensitivity gain is measured while positioning at a point where the position signals PosN and PosQ are equal. However, since it is measured on the apparatus, a measurement result is influenced by a fluctuation writing upon recording of the position signals, a fluctuation of a motor, and the like, so that the position fluctuates by about ±10%. As other factors, there are a fluctuation which is caused when a servo signal is recorded onto a disk medium, particularly, a deviation of a track width, an influence of noises, a quality of the signal on the medium at the measuring point, and noises of a demodulating circuit. Consequently, it is impossible to measure the position sensitivity gain which is perfectly correct and the measurement result is certainly accompanied with an error.
FIG. 10 shows a relation between the real position and the decoded position in the case where the position sensitivity gain is deviated by −20%. FIG. 11 shows a relation between the real position and the decoded position in the case where the position sensitivity gain is deviated by +20%. If the position sensitivity gain has a measurement error as mentioned above, a stairway occurs at a boundary of an interval which is demodulated by the position signal PosN and an interval which is demodulated by the position signal PosQ. As an influence accompanied by the error of the position sensitivity gain as mentioned above, there are the following two influences. The first one is a deterioration of positioning precision which is caused upon positioning of the head. Particularly, in the magnetic disk apparatus, a reading device and a writing device of a magnetic head are physically separated and a positional deviation occurs between them. Therefore, a target position upon writing and a reading position where the written signal is read out have to be deviated. Such a situation is called a read offset. The read offset has to be set to a different value every track or every zone in which a plurality of tracks are grouped into one region in accordance with dimensional shapes of the reading device and the writing device of the magnetic head which is mounted in the apparatus and in accordance with a yaw angle of a rotary actuator. Therefore, the target position is set to a point near the boundary of the position signals PosN and PosQ in dependence on the position of the read offset. At this time, if an error of the position sensitivity gain occurs, a deviation occurs between the position demodulated by the PosN and the position demodulated by the PosQ. Therefore, if the head is positioned at such a place, the positioning precision deteriorates and it is likely that the information recorded as data is erroneously read. The second one is precision of a detected position itself. A phenomenon such that the position sensitivity gain changes by 20% as shown in FIGS. 10 and 11 denotes that the detected position changes by 20% as a result. In the positioning control of the magnetic disk apparatus, it is an object to allow the positioning precision to lie within a target specification. However, if the precision of the detected position itself is uncertain, whether the specification itself could be satisfied or not cannot be guaranteed. As mentioned above, according to the conventional position demodulating method, although a condition that the position sensitivity gain is perfectly correct is set as a prerequisite, so long as its value cannot be correctly measured but has an error, the occurrence of the stairway at the demodulation boundary of the position signals PosN and PosQ cannot be avoided.